使用LINQ进行左连接

Question

请问有没有人能够给我一个使用LINQ/lambda表达式执行左连接操作的示例？

- Balamurugan

Answer 1

在MSDN上的LINQ to SQL samples page提供了一个示例，说明如何实现这一点。对于LINQ to Objects，代码应该基本相同。

关键在于调用DefaultIfEmpty。

Dim q = From e In db.Employees _
        Group Join o In db.Orders On e Equals o.Employee Into ords = Group _
        From o In ords.DefaultIfEmpty _
        Select New With {e.FirstName, e.LastName, .Order = o}

如果您需要将其转换为C#，请随时提问。

- Noldorin

Answer 2

这是在LINQ中进行左连接的一个示例。

- Reed Copsey

Answer 3

我尝试复制著名的左连接，其中b键为空，我得到的结果是这个扩展方法（稍加想象，您可以修改它以使其成为左连接）：

    public static class extends
{
    public static IEnumerable<T> LefJoinBNull<T, TKey>(this IEnumerable<T> source, IEnumerable<T> Target, Func<T, TKey> key)
    {
        if (source == null)
            throw new ArgumentException("source is null");

        return from s in source
               join j in Target on key.Invoke(s) equals key.Invoke(j) into gg
               from i in gg.DefaultIfEmpty()
               where i == null
               select s;
    }
}

- Totodile

Answer 4

例如：

IQueryable<aspnet_UsersInRole> q = db.aspnet_Roles
                    .Select(p => p.aspnet_UsersInRoles
                        .SingleOrDefault(x => x.UserId == iduser));

将为您提供来自asp.net成员资格的角色列表，其中包含与指定用户（iduser键）不匹配的空值

- Francisco

Answer 5

我发现我喜欢的方法是将 OuterCollection.SelectMany() 与 InnerCollection.DefaultIfEmpty()结合使用。您可以在“C# 语句”模式下使用 LINQPad运行以下内容。

var teams = new[] 
    { 
        new { Id = 1, Name = "Tigers" }, 
        new { Id = 2, Name = "Sharks" }, 
        new { Id = 3, Name = "Rangers" },
    };

var players = new[] 
    { 
        new { Name = "Abe", TeamId = 2}, 
        new { Name = "Beth", TeamId = 4}, 
        new { Name = "Chaz", TeamId = 1}, 
        new { Name = "Dee", TeamId = 2}, 
    };

// SelectMany generally aggregates a collection based upon a selector: from the outer item to
//  a collection of the inner item.  Adding .DefaultIfEmpty ensures that every outer item
//  will map to something, even null.  This circumstance makes the query a left outer join.
// Here we use a form of SelectMany with a second selector parameter that performs an
//  an additional transformation from the (outer,inner) pair to an arbitrary value (an
//  an anonymous type in this case.)
var teamAndPlayer = teams.SelectMany(
    team => 
        players
        .Where(player => player.TeamId == team.Id)
        .DefaultIfEmpty(),
    (team, player) => new 
        { 
             Team = team.Name, 
             Player = player != null ? player.Name : null 
        });

teamAndPlayer.Dump();

// teamAndPlayer is:
//     { 
//         {"Tigers", "Chaz"},
//         {"Sharks", "Abe"},
//         {"Sharks", "Dee"},
//         {"Rangers", null}
//     }

在尝试这个的时候，我发现有时可以省略匿名类型实例化中对player的空值检查。我认为这种情况是在使用LINQ-to-SQL访问数据库时出现的（而不是这里使用的数组，我认为这使它成为了LINQ-to-objects或其他什么）。我认为在LINQ-to-SQL中省略空值检查是可行的，因为查询被转换为SQL LEFT OUTER JOIN，直接跳过将null与外部项连接的步骤。（请注意，匿名对象属性的值必须是可空的；因此，如果您想安全地包含一个int，您需要像这样写：new { TeamId = (int?)player.TeamId }。

- Carl G